vscode-xml: Formatting breaks attributes containing quotes

How to reproduce:

1. Use this document: <hello attr='"' />
2. Run the “Format document” command, with xml.format.quotations set to doubleQuotes (the default)

Symptoms: The formatting produces the following invalid XML: <hello attr=""" />

Expected: Formatting shouldn’t change the type of quote used for an attribute if that produces invalid XML, that is if the attribute value contains the quote character.